The present invention relates to signal generators used in radar systems.
Signal generators that are used to produce transmitter drive and receiver local oscillator (LO) signals for tactical airborne radar systems are unique in the severity of the requirements placed on them for frequency stability, spectral purity, frequency agility, operating bandwidth, and small packaged volume. Prior techniques could generally optimize performance in one or more of these areas but at the expense of performance compromises in other areas. For example, a generator might provide increased band coverage but at the expense of increased spurious signal levels or increased hardware complexity and volume.
Prior techniques for generating the high quality signals needed for radar application have generally been compromises of optimizing some performance parameters at the expense of less performance for others. A common prior technique uses either a frequency comb generator or a programmable harmonic phase locked loop to provide coarse, approximately 100 MHz step size, frequency control across the operating band. A second phase locked loop that employs a frequency offsetting mixer is used to combine that coarse frequency control with fine step frequency selection provided by either a bank of crystal reference oscillators or a UHF or L-band frequency synthesizer. The resulting signal functions as the receiver first LO and as the reference to a second mixer. That mixer, which is also driven by a fixed frequency signal at the receiver first intermediate frequency (IF), produces the transmitter drive (TD) signal with desired offset from the first LO. This signal generation technique has provided excellent performance for narrow (less than 10 percent) operating bandwidths. For wider band operation, however, spurious signal control of the TD upconverting mixer becomes unmanageable and regions in the band must be designated as unsuitable for radar operation. The complete separation of the functions of coarse frequency generation, fine frequency generation, combining of the coarse and fine frequency control, and TD frequency offsetting facilitates function optimizations but results in a volume consuming design that is not compatible with most next generation applications. Finally, fault-tolerance, which can only be provided by duplication of the hardware, is generally not feasible from volume considerations.
Other techniques of frequency generation include indirect frequency synthesis, direct frequency synthesis, and direct digital frequency synthesis. Any one of these techniques could, in principal, be used to separately generate the TD and first LO and avoid the use of a TD offset mixer and its associated spurious signal problems. Unfortunately, each of these approaches, when used alone, suffers from one or more shortcomings.
Indirect frequency synthesis is accomplished using a phage locked loop with wide range frequency dividers in the reference and the feedback signal paths to control the output frequency. Although relatively simple, divider noise severely restricts their use in a radar application.
Direct frequency synthesis uses an arrangement of mixers, frequency dividers, frequency multipliers, and filters to derive desired output frequencies from arithmetic combinations of one or more reference frequencies. These synthesizers can provide extremely fast switching but are generally very volume consuming when a large number of channels are required.
Direct digital frequency synthesis techniques employ a digital sine/cosine look-up table memory, suitable high speed control, clocking, and memory addressing circuitry, and a D/A converter to directly construct frequency programmable sine and cosine voltage waveforms. These frequencies are upconverted to the desired operating band by conventional analog techniques. This technique is extremely powerful in its ability to generate closely spaced channel frequencies and to simultaneously impose modulations but is limited with today's technology to poor spurious signal and FM noise performance.
None of the above approaches to radar signal generation offer as clean a way of generating both the TD and the first LO over a wide operating band as the hybrid approach of the present invention. Additionally, none provide the inherent fault-tolerance of the invention, a feature that is believed to be unique for the radar microwave signal generation function.